This invention relates to an integrator circuit for integrating input signals having the form of sampled electrical currents, the integrator having an input means for receiving an input signal and an output for making available an output signal and comprising first and second current memory cells, means for connecting the output of the first current memory cell to the input of the second current memory cell, means for connecting a first output of the second current memory cell to the input of the first current memory cell, means for connecting the input means of the integrator to the input of the first and/or second current memory cell, and means for connecting a second output of the second current memory cell to the output of the integrator, wherein the first current memory cell is arranged to produce at its output during a first portion of each sampling period a current related to that applied to its input during a second portion of the preceding sample period and the second current memory cell is arranged to produce at its outputs during a second portion of each sampling period currents related to the current applied to its input during the first portion of the sampling period.
Such an integrator circuit has been disclosed in UK Patent Applications Nos. 8721758 (PHB33385), 8721759 (PHB33386), 8729987 (PHB33426), 8828666.1 (PHB33512), and 8828667.9 (PHB33514), which correspond to U.S. Pat. Nos. 4,864,217 (9/5/89), 4,866,368 (9/12/89), 4,897,596 (1/30/90), U.S. Pat. No. 5,021,692 (6/4/91), and U.S. application Ser. No. 446,821 (12/6/89), respectively, the contents of which are hereby incorporated by reference. Further, such an integrator circuit has been disclosed in a paper presented at the Institution of Electrical Engineers on 17th February, 1989 by J. B. Hughes, N. C. Bird, and I. C. Macbeth entitled "Switched Currents--A New Technique for Analogue Sampled Data Signal Processing". The integrator circuit is one of a number of modules which have been devised for use in circuits using the technique disclosed which has been called "switched current techniques" and the circuits using this technique have been called "switched current circuits" and will be referred to hereinafter by that name. As has been disclosed in the patent applications and paper referred to hereinbefore, switched current circuits have certain advantages over other sampled data processing circuits such as switched capacitor circuits, but are able to use many of the mathematical techniques used for the design of switched capacitor circuits.
In the design of continuous time active R.C. filters and switched capacitor filters a feed forward facility is often required in biquaratic and lossless ladder filter synthesis. This is easily achieved in active R.C. filters and also in switched capacitor filters where it can utilise non-switched capacitors. However, in switched current circuits there is no provision for accurately defined linear capacitors and hence such capacitive feedforward facilities cannot be provided.